Focus on Russia: fuel design
VVER fuel update26 August 2011
Russia now supplies almost a fifth of the world’s nuclear fuel. TVEL’s fuel development programmes for Russian-designed VVER light water reactors in Russia and abroad aim to increase the service life, burn-up and cost effectiveness of its fuel assemblies. By P.I. Lavrenyuk and V.L. Molchanov
Russian fuel manufacturer TVEL currently accounts for 17% of the world nuclear fuel market. The company supplies fuel for research, transport and commercial power reactors worldwide. Most of TVEL’s business is in Russia, where it supplies fuel to state-operator Rosenergoatom for the country’s 32 reactors (VVER-440, VVER-1000, RBMK-1000, BN-600 and EGP-6 designs). But it also manufactures fuel for VVER reactors in Armenia, Bulgaria, Czech Republic, China, Finland, Hungary, India, Iran, Slovakia, and Ukraine, and for foreign PWR and BWR designs.
TVEL aims to produce nuclear fuel that meets its consumers’ requirements and remains competitive, while ensuring all the necessary safety standards are met during its manufacture and use.
Achievement of these goals requires the constant upgrading of existing fuel assembly designs to increase their service life, improve their operational reliability, and to introduce safe and more cost effective fuel cycles. Development goals include achieving increased fuel burn-up, extending fuel life (to 18 months, for example) and reducing the neutron load on the reactor vessel.
Today, the main fuel used in advanced VVER-440 (V-213) reactors is second-generation fuel known as TVS ARK. This fuel is being used in a five-year fuel cycle at units 3&4 of Russia’s Kola NPP, units 1-4 of Dukovany NPP (Czech Republic), and units 3&4 of Bohunice NPP and units 1&2 of Mohovche NPP in Slovakia.
For first-generation VVER-440 reactors
(V-179, V-230 and V-270), vibration-proof working fuel assemblies (WFAs) have been developed and are being successfully used in a four-year fuel cycle at units 1&2 of Kola NPP. These WFAs are based on second-generation FA technology, but take into account the additional exposure to heat carrier flow associated with first generation VVER-440s.
Current work on VVER-440 fuel involves the development of higher-enriched fuel (enriched to 4.87% uranium-235, rather than the current 4.38%). A plan for introducing this fuel into unit 4 of Kola NPP was worked out in 2010; the programme is currently in trial operation. TVEL is planning to study the use of the higher-enriched fuel at Slovakia’s Bohunice and Mohovche NPPs beginning in 2011; the fuel is now on its way to site.
A third-generation VVER-440 fuel has been introduced with a shroudless skeleton. The new structure incorporates angle elements, guiding thimbles and increased fuel rod pitches up to 12.6 mm in the bundle to improve the water-uranium ratio. These modifications are expected to increase fuel utilization effectiveness by 10%. The technical project to develop third generation fuel began in 2007 and in 2010 a trial batch was supplied to Kola NPP for pilot operation.
Due to their ages, Novovoronezh units 3-5 (V-179, V-179 and V-187) will continue to receive older models of fuel.
Stages of VVER-440 nuclear fuel development, the characteristics of the working fuel assemblies and fuel cycles are shown in Table 1.
TVEL currently supplies three different designs of VVER-1000 fuel assembly: TVS-2M for Balakovo and Rostov NPPs; TVSA-PLUS for Kalinin NPP; and TVSA for the units in Ukraine and Bulgaria. Table 2 shows the developments in VVER-1000 fuel design, and gives technical characteristics of these three designs.
The newer-generation fuel assemblies (TVSA-PLUS, in Figure 2, and TVS-2M) offer increased uranium capacity, improved heat reliability and enhanced operational safety.
TVSA-PLUS and TVS-2M have improved geometric stability during operation due to a radiation-resistant zirconium alloy (E-635) wireframe, which makes bending or twisting of the assembly almost impossible. The core thermal hydraulic reliability is improved. The newer-generation fuels also have on-site repairable demountable designs (removable heads for extraction of fuel rods), and the possibility of rod cluster control assembly (RCCA) malfunctions is prevented.
The improved FA design facilitates quicker transport times between the reactor core and the spent fuel pools, thereby reducing the duration of refuelling operations.
The improved geometry has also enabled a number of other improvements, including:
• Development of a new generation of zirconium-alloy fuel rods filled with UO2 and Gd2O3 pellets. Use of U-Gd rods improves power density distribution along the fuel assembly and enhances core safety
• Increase fuel assembly uranium loading
• Load following capability – with daily operation in the range 100-75-100% nominal capacity (Nnom)
• Safe performance under power uprate conditions of up to 104% Nnom and over
• Implementation of ‘in-in-out’ core loading patterns taking into account new design power peaking factors
• Extended fuel cycle lengths.
The uranium content of the fuel assemblies has been increased in two different ways: by increasing the diameter of the fuel pellets in fuel rods, while simultaneously reducing the cladding thickness; and increasing the fuel column height while maintaining the overall dimensions of the cartridge.
TVSA-ALFA fuel rods, for example, have a cladding thickness of 0.57mm (instead of 0.67mm) and pellets of 7.8mm diameter without a central hole (instead of 7.6mm in diameter with a 1.2mm diameter hole). The design of TVSA-ALFA includes 12 spacer grids (instead of 15). These measures increase the fuel mass by approximately 10%.
In TVS-2M fuel rods, the fuel column height has been increased by 150mm and the fuel pellets measure 7.6/1.2mm, which has increased the fuel mass by around 6%. Both designs include anti-vibration grids and since 2010 have included anti-debris filters (which retain particles >2 mm), and 3 mixing grids.
Since 2008, TVSA-ALFA and TVS-2M fuel have been exclusively used in Kalinin 1 and Balakovo 1, respectively. Kalinin 1 NPP is now operating on a five-year fuel cycle (5 × 320 effective full power days?(EFPDs), while unit 1 of Balakovo NPP operates on an 18-month cycle (3 × 510 EFPDs).
In 2010, the remaining units of Balakovo and both Rostov units switched to using TVS-2M fuel on an 18-month operating cycle. The fuel change has allowed the reactors to operate at an uprated capacity of 104%.
TVSA-PLUS fuel, based on the TVS-2M design, has been designed for units 2&3 of Kalinin NPP. From 2010 these units began using TVSA-PLUS fuel in an 18-month fuel cycle, again operating at a capacity of 104%.
Special fuel, TVSA-T, is also being supplied to Temelin NPP in the Czech Republic for a ten-year period, beginning in 2010 [see also NEI June 2010, pp.41-42].
The fuel requirements for Russia’s next generation VVER-1200 reactors (also known as NPP-2006 projects) differ from those of VVER-1000s. The thermal capacity of the VVER-1200, 3200-3300 MWt, is up to 10% greater than the VVER-1000, the steam temperature at the reactor inlet, 299°C, is nine degrees hotter, the maximum temperature of the fuel rod cladding, 359°C, is seven degrees hotter, and the steam void fraction in a hot channel as a percentage of weight, 13%, is 2.6 times larger.
Fuel assemblies for VVER-1200 reactors will be based on the evolution of existing technology. The fuel should be capable of use during longer fuel cycles (up to 24 months), have increased thermal capacity, allow operation in load-following mode and include demountable fuel assemblies, which give the option to replace defective fuel rods.
Based on these requirements, programmes for VVER-1200 fuel design include:
• Development and validation of using fuel assemblies with intermediate flow mixers (IMF) for increasing departure from nucleate boiling ratio (DNBR) margin and reducing void fraction in the hot channel
• Substantiation of corrosion and radiation resistance of fuel rods at elevated temperature, pressure and steam content (compared to VVER-1000)
• Development and validation of modified alloys, and the start of their production
• Calculation and experimental substantiation (including in-pile testing in MIR reactor) of actual fuel characteristics during power manoeuvring
• Development of an inspection and fuel assembly repair stand that enables FA inspection during reactor power operation.
The first stage TVS-2006 fuel development (basic design) took place from 2007 to 2009. This fuel will be supplied to Novovoronezh II.
Fuel cycles are based on either 3x18 months or 5x12 months. The 3x(510) cycle has 72 FAs in the reload batch, average enrichment of 4.69%, an average FA burn-up of 48.3 MWd/kgU and a lead rod burn-up of 57 MWd/kgU. The 5x(310) cycle has 36 FAs in the reload batch, average enrichment of 4.83%, an average FA burn-up of 58.4 MWd/kgU and a lead rod burn-up of 64.2 MWd/kgU. Pellet size is 7.6mm with 1.2mm hole; the grain size is 25-30 µm and power manoeuvring capability is in the range of 100-75-100% Nnom).
The second stage of fuel development, scheduled for completion in 2013, has begun. Initiatives include expanding the range of load-follow; increasing uranium load and introducing fuel rods with thinned (0.57 mm)?cladding; conducting FA pilot operation with flow mixers; validating high-burnup fuel rods in LOCA?and RIA design accidents, including fuel rods containing pellets without a central hole; developing and introducing advanced zirconium alloys.
TablesTable 1: Development of VVER-440 nuclear fuel Table 2: Development of VVER-1000 nuclear fuel